1. Field of the Invention
The present invention relates to an organic light emitting diode display, and more particularly to an organic light emitting diode display and a method of driving the same capable of increasing the display quality by preventing a driving current from becoming degraded by the degradation of a drive thin film transistor (TFT) depending on driving time.
2. Discussion of the Related Art
Recently, various kinds of flat panel display devices with reduced weight and size have been developed as a replacement of cathode ray tubes. Examples of the flat panel display devices include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP), and electroluminescence devices. Because the structure and manufacturing process of plasma display panels are simple, the plasma display panels have been considered for large-sized display devices that are relatively light and thin. However, the emitting efficiency and luminance of the plasma display panel are low while its power consumption is high. As an alternative, thin film transistor (TFT) LCD using TFTs as a switching device is widely used. However, the TFT-LCD is a non-emitting device. Therefore, the TFT-LCD has a narrow viewing angle and a low response speed. The electroluminescence device, on the other hand, is a self-emitting device. The electroluminescence device may be classified into an inorganic light emitting diode display category and an organic light emitting diode (OLED) display category depending on the material of an emitting layer. Because the OLED display includes a self-emitting device, the OLED display has high response speed, high emitting efficiency, strong luminance, and wide viewing angle.
An OLED display includes an organic light emitting diode. As shown in FIG. 1, the organic light emitting diode includes organic compound layers 78a, 78b, 78c, 78d, and 78e between an anode electrode and a cathode electrode. The organic compound layers include an electron injection layer 78a, an electron transport layer 78b, an emitting layer 78c, a hole transport layer 78d, and a hole injection layer 78e. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer 78d and electrons passing through the electron transport layer 78b move to the emitting layer 78c to form an exciton. Hence, the emitting layer 78c generates visible light.
The OLED display is arranged with pixels including the organic light emitting diode in a matrix format and controls brightness of the pixels selected by a scan pulse depending on a gray level of digital video data. The OLED display may be classified into a passive matrix type OLED display and an active matrix type OLED display using a thin film transistor as a switching device. In particular, the active matrix type OLED display selectively turns on the thin film transistor used as the switching device to select the pixel and maintains an emission of the pixel using a voltage hold by a storage capacitor.
FIG. 2 is an equivalent circuit diagram showing one pixel in a related art active matrix type OLED display. As shown in FIG. 2, an pixel of the related art active matrix type OLED display includes an organic light emitting diode OLED, data lines DL and gate lines GL that cross each other, a switching thin film transistor SW, a drive thin film transistor DR, and a storage capacitor Cst. The switch TFT SW and the drive TFT DR may be an N-type metal-oxide semiconductor field effect transistor (MOSFET).
The switching TFT SW is turned on in response to a scan pulse received through the gate line GL, and thus a current path between a source electrode and a drain electrode of the switching TFT SW is turned on. During on-time of the switching TFT SW, a data voltage received from the data line DL is applied to a gate electrode of the drive TFT DR and the storage capacitor Cst via the source electrode and the drain electrode of the switching TFT SW. The drive TFT DR controls a current flowing in the organic light emitting diode OLED depending on a voltage difference Vgs between the gate electrode and a source electrode of the drive TFT DR. The storage capacitor Cst stores the data voltage applied to an electrode at one end of the storage capacitor Cst to keep a voltage applied to the gate electrode of the drive TFT DR constant during a frame period.
The organic light emitting diode OLED may have a structure shown in FIG. 1. The organic light emitting diode OLED is connected between the source electrode of the drive TFT DR and a low potential driving voltage source VSS. A brightness of the pixel shown in FIG. 2 is proportional to the current flowing in the organic light emitting diode OLED as indicated in the following Equation 1:
      Vgs    =          Vg      -      Vs                                                Vg            =            Vdata                    ,                                      Vs          =          Vss                          Ioled    =                            β          2                ⁢                              (                          Vgs              -              Vth                        )                    2                    =                        β          2                ⁢                              (                          Vdata              -              Vss              -              Vth                        )                    2                    
In the above Equation 1, Vgs indicates a voltage difference between a gate voltage Vg and a source voltage Vs of the drive TFT DR, a data voltage Vdata, a low potential driving voltage Vss, a driving current Ioled, a threshold voltage of the TFT DR Vth, and a constant β determined by mobility and parasitic capacitance of the drive TFT DR.
As indicated in the above Equation 1, the driving current Ioled of the organic light emitting diode OLED is greatly affected by the threshold voltage Vth of the drive TFT DR. When the gate voltages with the same polarity are applied to the gate electrodes of the drive TFT DR for a long time, a gate-bias stress and the threshold voltage Vth of the drive TFT DR increases. Hence, operation characteristics of the drive TFT DR change over time. The changes in the operation characteristics of the drive TFT DR can be seen from an experimental result shown in FIG. 3.
FIG. 3 is a graph showing changes in operation characteristics of hydrogenated amorphous silicon TFT sample (A-Si:H TFT) when a positive gate-bias stress is applied to the hydrogenated amorphous silicon TFT sample (A-Si:H TFT) whose channel width to channel length ratio W/L is 120 μm/6 μm. In FIG. 3, the transverse axis indicates a gate voltage of the A-Si:H TFT, and the vertical axis indicates a current between a source electrode and a drain electrode of the A-Si:H TFT.
More specifically, FIG. 3 shows a threshold voltage of the A-Si:H TFT depending on voltage application time and a movement of the transmission characteristic curve when a voltage of 30 V is applied to a gate electrode of the A-SI:H TFT. As can be seen from FIG. 3, as application time of a positive voltage to the gate electrode of the A-Si:H TFT becomes longer, the transmission characteristic curve of the A-Si:H TFT moves to the right of the graph shown, and the threshold voltage of the A-Si:H TFT rises from a voltage Vth1 to a voltage Vth4.
A rise level of the threshold voltage of the A-Si:H TFT depending on the voltage application time changes in each pixel. For example, a rise width of a threshold voltage of a drive TFT in a first pixel to which a first data voltage is applied for a long time is smaller than a rise width of a threshold voltage of a drive TFT in a second pixel to which a second data voltage larger than the first data voltage is applied for a long time. In this case, the amount of driving current flowing in an organic light emitting diode generated by the same data voltage in the first pixel is more than that of the second pixel. Hence, the display quality is deteriorated.
A method in which a rise in the threshold voltage of the drive TFT is suppressed by applying a negative gate-bias stress to the drive TFT was recently proposed to prevent the deterioration of the display quality. However, it is difficult to completely compensate for a difference between driving currents of the pixels by only applying a negative voltage as pixel data to suppress the rise in the threshold voltage of the drive TFT. As indicated in the above Equation 1, the driving current Ioled flowing in the organic light emitting diode is affected by a potential value of a Vss supply line for supplying the low potential driving voltage Vss and the mobility of the drive TFT DR determining the constant β as well as the threshold voltage of the drive TFT DR. When the driving current flows in each pixel of an OLED display panel, the low potential driving voltage Vss changes depending on a location of the pixel because of a resistance of the Vss supply line. The mobility of the drive TFT DR is also degraded depending on the driving time. Therefore, a difference between the threshold voltages of the drive TFTs DR, a potential difference between the Vss supply lines, and a difference between the mobilities of the drive TFTs DR have to be compensated so that the display quality is improved by reducing a deviation of the driving current of each pixel.